1. Field of the Invention
The present invention relates to a novel polymeric liquid-crystalline compound having an optically active group on its polymer main chain, a liquid-crystal composition comprising the polymeric liquid-crystalline compound, and a liquid-crystal device making use of them.
2. Related Background Art
Polymeric liquid-crystal materials have recently attracted notices as high-function materials. This is considered due to the fact that polymeric liquid crystals have the following superior characteristic features.
(1) They can be readily formed into thin films, and make it possible for liquid-crystal devices to have a large area, which has been difficult in low-molecular liquid crystals.
(2) Application of aligning treatment such as stretching can give a good state of alignment.
(3) A liquid-crystal state of polymers can be retained even at a temperature of not higher than the glass transition temperature, and hence the memory performance can be greatly improved, compared with the case in which low-molecular liquid crystals are used.
A problem arising when polymeric liquid crystals are put into practical use has been said to be a slow speed of response to an electric field. The speed of response to an electric field, however, can be considered to be greatly improved by the use of a ferroelectric polymeric liquid crystal having a spontaneous polarization as in low-molecular liquid crystals.
In order for a polymeric liquid crystal to exhibit a ferroelectricity, an optically active group must be introduced into a flexible spacer (a flexible chain).
With regard to what are called side-chain type polymeric liquid crystals having an acrylic or methacrylic main chain or the like, a report has already been made on ferroelectric polumeric liquid crystals having various molecular structures (see, for example, Japanese Patent Application Laid-Open No. 63-72784, No. 63-99204 and No. 63-161005).
On the other hand, main-chain type polymeric liquid crystals are superior in strength when they are formed into films, and also their molecular chains can be readily aligned by stretching or the like, compared with the side-chain type polymeric liquid crystal. As a result, they can be superior in view of, e.g., the fact that the films can be readily made to have large areas even when a low-molecular liquid crystal is contained.
Such a main-chain type ferroelectric polymeric liquid crystal having a bicyclic biphenyl group as a mesogen and an optically active group in the main chain has been reported in Collections of Lecture Drafts for The 14th Liquid Crystal Forum, pp.56-57, September 1988, Japanese Patent Application Laid-Open No. 64-65124.
What is disclosed in this publication, however, is a monotropic polymeric liquid crystal that renders a chiral smectic phase only when temperature is lowered, and it has been difficult for a stable liquid-crystal phase to be exhibited. in addition, because of use of a specific optically active group, this liquid crystal has a narrow range of selection in view of the control of temperature characteristics, optical characteristics and electrical characteristics and also has not been well satisfactory in view of, e.g., its electrical characteristics.